1. Field of the Invention
This invention generally relates to a surface acoustic wave device, and more particularly, to a surface acoustic wave device having a shield electrode and at least one interdigital transducer (hereinafter referred to as IDT) on a piezoelectric material substrate (hereinafter referred to as piezoelectric substrate).
2. Description of the Related Art
In these years, the above-mentioned type of surface acoustic wave (hereinafter referred to as SAW) has been employed for a bandpass filter in a television set having a frequency range of 30 MHz to 400 MHz and an RF filter in a mobile telephone having a frequency range of 800 MHz to several GHz. An IDT includes a pair of comb-like electrodes. Each comb-like electrode is composed of a bus bar and electrode fingers having first edges connected to the bus bar and second edges that are open. A pair of comb-like electrodes is arranged so that the electrode fingers of the comb-like electrodes are alternately crossed or interleaved at regular intervals. In other words, the interleaved electrode fingers are alternately connected two bus bars. A SAW is generated by applying an alternating voltage across the pair of comb-like electrodes. The SAW has a frequency response by which a filter having a desired frequency characteristic is obtainable.
FIGS. 1A and 1B show a conventional SAW device. FIG. 1A shows an overall schematic diagram of the conventional SAW device, and FIG. 1B shows an enlarged view of a shield electrode shown in FIG. 1A. The SAW device shown in FIGS. 1A and 1B includes a first IDT 10 that is arranged on a piezoelectric substrate 50, a shield electrode 20, and a second IDT 30. The first IDT 10, the shield electrode 20, and the second IDT 30 are adjacently arranged in the direction of SAW propagation. The shield electrode 20 is arranged between the first IDT 10 and the second IDT 30. For instance, the first IDT 10 serves as an input electrode, and the second IDT 30 serves as an output electrode. The shield electrode 20 prevents electromagnetic coupling of the first IDT 10 and the second IDT 30.
The IDT 30 includes a pair of comb-like electrodes 30a and 30b. The comb-like electrode 30a includes a bas bar and multiple electrode fingers. The comb-like electrode 30b also includes a bas bar and multiple electrode fingers. The open edges of the electrode fingers 30a face those of the electrode fingers 30b. The crossing portions of the interleaved electrode fingers that face each other are involved in excitation of SAW. As shown in FIG. 1B, an electrode finger pattern is weighted. The electrode finger pattern is defined as a pattern formed by the electrode fingers. The electrode finger pattern may by weighted by, for example, apodization. The weighting of the electrode finger pattern may alter the frequency characteristic.
The IDT 10 also includes a pair of comb-like electrodes; however, the IDT 10 is not weighted, which is different from the IDT 30. In other words, the electrode fingers of the IDT 10 have an equal crossing width.
The above-mentioned filter functions as a bandpass filter. This type of bandpass filter has a problem in that undesired waves are generated from the electrode finger edges of the IDT 30. The undesired waves degrade the frequency characteristic.
In order to remove the undesired waves, antireflection electrodes 40 are arranged on one side of the IDT 30 adjacent to the shield electrode 20, as shown in FIG. 1B. The antireflection electrodes 40 include multiple electrode fingers. The multiple electrode fingers are configured so that the undesired waves are generated so as to be equal in amplitude but opposite in phase. Thus, the undesired waves can be cancelled by each other. The above-mentioned conventional technique is disclosed in, for example, Japanese Laid-Open Patent Application Publication No. 57-25714 (hereinafter referred to as Document 1) or Japanese Laid-Open Patent Application Publication No. 59-125113 (hereinafter referred to as Document 2).
Japanese Laid-Open Patent Application Publication No. 10-41778 (hereinafter referred to as Document 3) discloses a technique to remove the undesired waves propagating inside the piezoelectric substrate. The publication shows the use of a dummy electrode arranged on one side of the IDT 30.
Further, Japanese Laid-Open Patent Application Publication No. 58-43608 (hereinafter referred to as Document 4) shows that the crossing portions of the IDT 30 has a tilt in arrangement, as shown in FIG. 2. The tilt arrangement prevents undesired bulk waves from being generated.
However, the techniques disclosed in Documents 1, 2, and 4 are intended to remove the undesired waves generated between the input IDT and a stem in the direction of the thickness of the piezoelectric substrate, or to remove the undesired waves generated solely at one side of the input IDT. Even if the above-mentioned undesired waves are removed, there is another problem in that a satisfactorily high attenuation in the stopband cannot be achieved. Besides, the technique disclosed in Documents 3 cannot remove the undesired excitation completely, since minute crossing portions exist on the side of the IDT 30. Thus, the undesired excitation cannot be removed completely, so that the signals cannot be sufficiently attenuated in the stopband.